It’s a common situation: you create a site and it’s ready to go. It’s all on GitHub. But you’re not really done. You need to set up deployment. You need to set up a process that runs your tests for you and you're not manually running commands all the time. Ideally, every time you push to master, everything runs for you: the tests, the deployment... all in one place.
Previously, there were only few options here that could help with that. You could piece together other services, set them up, and integrate them with GitHub. You could also write post-commit hooks, which also help.
But now, enter GitHub Actions.